The present invention relates generally to electrically-induced osteogenesis and more specifically to an improved method and apparatus for optimizing stimulated osteogenesis.
It is known in the prior art to apply a cathode of metal such as platinum (Pt), titanium (Ti) or stainless steel at a bone site and an anode at a skin or tissue location near the cathode implant. The signal is applied to pass currents between the anode and cathode. Bone formation is said to be particularly favorable at 20 microamperes for single or multiple cathodes as described in U.S. Pat. No. 3,842,841 to Brighton, et al.
More recently, it has been reported in U.S. Pat. No. 4,519,394 to Black, et al that optimum bone formation is assisted by maintaining a current in the range of 0.1 to 100 microamperes per cathode port and maintaining the cathode port at a voltage substantially constant in the range of 1.0 to 1.26 volts relative to a silver-silver chloride (Ag/AgCl) reference electrode implanted or contacting body tissue. As indicated in FIG. 1 this is a three electrode system including a percutaneous or implanted cathode 18 having a port 20 positioned at a tissue site 12 of a bone 10. A transcutaneous anode 22 may be placed on the skin 16 or fully implanted in muscle or other convenient tissue. A percutaneous or implanted reference electrode 30 having a port 32 is inserted into the living tissue 14 at a point remote from the cathode and anode locations.
The current between cathodes of materials such as stainless steel, platinum, titanium or carbon and an appropriately chosen anode rises slowly with applied voltage until a voltage zone is reached at which the current increases more rapidly for small increases in voltage. This transition region ("knee") of the current-voltage characteristic or curve corresponds with the onset of chemical reactions such as oxygen reduction and hydroxide ion formation in the region of the cathode. Typically the knee occurs at an inter-electrode voltage of about 2.4 volts in physiological conditions for anode-cathode pairs such as stainless steel--stainless steel. The position of the knee also depends on tissue impedance (which changes over time) and electrode position, among other variables.
It is also known from animal experiments that bone accretion occurs at the cathode and that overly large currents cause bone loss and necrosis due to local formation of amounts of electrode reaction Products in excess of the ability of the tissue region to absorb and disperse them. There is also evidence that with particular cathodes such as stainless steel or titanium, the entire current may pass through a region close to the end of the insulation of the lead accessing the treatment site. Correspondingly, the finding in animals that 20 microamperes is optimal for the tested cathodes and cathode geometries will not describe optimum stimulation for other cathodes and geometries. Furthermore, the prior methods discussed above for maintaining the cathode voltage in a fixed range relative to a reference electrode does not necessarily optimize the voltage-current relationships with respect to the growth process and have required three electrodes.
Thus it is an object of the present invention to provide a two electrode system which provides an optimization of the current-voltage for an osteogenic stimulation.
Another object of the present invention is to provide an apparatus and method for optimising osteogenic stimulation which adapts for variation in the tissue impedance and cathode properties over time.
These and other objects are achieved by applying varying signals to a first electrode at the tissue site and a second electrode remote from the tissue site and monitoring the results to determine a distinctive transition (knee) in the current-voltage characteristics of the pair of electrodes. A signal is then selected and applied to the electrodes to operate beyond the transition. Periodically, varying signals are applied to the two electrodes and the monitoring process reperformed to determine a new transition and the appropriate signal is selected to operate beyond the transition. The current between the electrodes is typically between 10 and 50 microamps and an appropriate voltage is selected to operate beyond the transition. In some tissue repair situations it will be useful to use more than one cathode implanted in separate positions within the repair region, and each cathode may be optimised independently as described for single cathodes. In other circumstances a branched or multiport cathode may be convenient, and currents typically between 10 and 50 microamps per branch or port may be chosen at potentials beyond the transition as determined for the assembly.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.